1. Field of the Invention
The invention relates to a process and apparatus for optimizing, characterizing and checking a crack detection analysis method.
The invention is applicable to the detection of surface defects in all kinds of articles by the crack detection method, and more particularly to the automation, characterization and optimization of crack detection methods.
2. Summary of the Prior Art
In the known examination methods using crack detection the article to be examined is first prepared so as to reveal the surface defects. The preparation consists of applying to the surface of the article a penetrating composition, also called an indicator product, which contains a fluorescent or coloured dye and which penetrates into the openings of the defects on the article surface, then removing the excess penetrating composition from the surface before applying a developing composition. The developing composition acts as a blotting paper and works by capillary action to attract the penetrating liquid composition containing the fluorescent dye out of the surface defects. The article is then exposed to appropriate lighting conditions such as an invisible fluorescent-producing light, and the presence of the surface defects is revealed by the emission of a visible fluorescent light by the dye of the penetrating composition remaining in the cracks after the removal of the composition from the article surface. As a rule, the penetrating composition contains fluorescent pigments which, when exposed to light in the ultraviolet range, re-emit a visible light usually disposed in the green-yellow colour range. In this case the method is called the fluorescent crack detection method.
These methods of displaying defects provide a convenient way for checkers to examine articles visually. This visual evaluation is a difficult job and is very tiring for the eyes of the checking personnel, especially when articles produced in large series have to be checked, which affects concentration. Also, it is difficult to eliminate subjective influences from visual judgements.
Automatic examination of the indicators given by crack detection methods has also been proposed. For this purpose, it is known to use for the automatic collection and evaluation of defect indications opto-electronic systems comprising means for illuminating an image field in which the article to be checked is placed, video means such as a camera for obtaining digital images of the article, and image-processing means such as a computer for detecting possible surface defects in the article by comparison with images of reference articles.
However, the sensitivity and discrimination of the known automatic methods are usually limited by a number of factors, in particular the lighting conditions of the article to be checked and the pre-check preparation conditions of the article.
In the case of fluorescent crack detection methods illumination is provided by a light source which emits ultraviolet radiation. The radiation from the light source is filtered by filters to select a range of wavelengths appropriate to the particular penetrating composition chosen. Filters are also provided at the entry to the camera to eliminate parasitic radiations not originating from the penetrating composition. The use of these filters is not entirely satisfactory since they consume energy and increase the size of the automatic checking apparatus. Also, the filters do not have a pass band adapted to this kind of use and, in particular, it is very difficult to eliminate parasitic wavelengths around 400 nanometers. These wavelengths, which are disposed at the limit of the visible spectrum, usually have a very high energy. They pollute the images by causing reflections and affect the reliability of the checking result by masking crack information and causing parasitic indications called phantom defects.
Also, the known automatic methods are usually based on comparison with reference images or articles and give only qualitative results of uncertain reliability since they depend upon numerous parameters determined empirically by the user.
Checking reliability depends inter alia upon the article-preparing process preceding examination of the article, and in particular upon the choice of penetrating composition and upon the conditions in which the article is cleaned to remove excess penetrating composition. The reliability of the result also depends upon the camera and upon the processing of the images obtained by the video means.